In variable air volume (VAV) heating, ventilation and air conditioning systems, the flow of air through a duct system is varied to provide thermal comfort and proper ventilation. Multi-point, center-averaging sensors are typically provided at the inlets of terminal boxes in VAV systems to measure the total pressure and static pressure produced by air flowing through the duct system. The difference between these two measurements, known as "differential pressure", is proportional to the flow of air through the duct system. The total and static pressure measurements, which are preferably averaged and amplified by the sensor, are transmitted to a controller which responds by increasing or decreasing the rate of air flow through the duct by opening or closing a damper located downstream from the sensor.
Typical examples of multi-point, center-averaging airflow sensors are disclosed in U.S. Pat. No. 4,453,419 (Engelke) and U.S. Pat. No. 5,481,925 (Woodbury). The sensor disclosed by Engelke comprises a number of pairs of tubes extending radially outwardly from a central hub. One tube of each pair has upstream facing holes and transmits a total pressure signal to a first chamber in the central hub, while the other tube of each pair has downstream facing holes and transmits a static pressure signal to a second chamber in the central hub. The total pressure signals and static pressure signals are averaged and amplified in the respective chambers of the hub and are then transmitted to the controller. The Woodbury sensor is similar to that of Engelke, however the total and static pressure sensing tubes are not paired and only a single static pressure port is provided at the radially outer end of the static pressure sensing tubes.
Most known airflow sensors operate on a principle similar to that of a standard Pitot tube, traversing the air stream and measuring the pressure at a number of locations in the duct. Such sensors are capable of generating reasonably accurate pressure measurements when the airflow pattern through the duct is substantially symmetrical, that is when the velocity of air flowing through the duct exhibits a parabolic distribution across the duct section, with the maximum velocity at the center of the duct and the minimum at the walls of the duct. While symmetrical airflow conditions may exist in relatively long, straight portions of the duct system, the presence of obstructions such as elbows, transitions, flexible ducts and dampers in the duct system cause turbulence which disrupt the symmetrical airflow pattern, and have a negative impact on the accuracy of the pressure measurements generated by the duct. This problem has typically been dealt with in the past by spacing the airflow sensor several duct diameters downstream from any elbows or the like, and several duct diameters upstream from dampers. The inventors have tested a number of known sensors and have found no sensor design which is immune to measurement errors resulting from asymmetric airflow.
Another problem associated with known airflow sensors is that the sensor itself is responsible for a certain amount of turbulence in the duct system. Sensor-generated turbulence causes a drop in pressure across the sensor as well as noise in the duct system. This problem has been dealt with in the prior art, for example in the Woodbury patent, by streamlining and minimizing the surface area of the sensor in order to reduce both turbulence and noise. More recently, this problem has also been addressed by eliminating the static pressure sensing tubes altogether and providing the sensor with only one static pressure port located on the rear surface of the hub. However, the inventors have discovered that back pressure in the duct system caused by a damper located downstream from the sensor causes inaccuracies in the static pressure signal generated by such a sensor.
Airflow sensors are typically supplied by manufacturers as assemblies comprising a sensor installed in either a short sheet metal sleeve or a terminal box to be incorporated into the duct system. When such assemblies are manually moved from place to place on a job site, it is not uncommon for the assembly to be lifted and carried by grasping one of the arms of the sensor. Due to the fact that the sensor tubes are typically of small diameter to reduce sensor surface area, they are easily damaged by such handling, the result being that the measurements generated by the sensor may be inaccurate. This problem has persisted despite warning labels affixed to sensor/duct assemblies by manufacturers.